The present invention concerns a longitudinal magnetic recording medium, such as magnetic recording drums, magnetic recording tapes, magnetic recording disks and magnetic recording cards, as well as a magnetic storage apparatus and, more particularly, the invention relates to a longitudinal magnetic recording medium which enables super high density recording at 3 Gbit or more per one square inch, and a magnetic storage apparatus using the longitudinal magnetic recording medium.
In recent years, the sizes of recording bits formed on a magnetic recording medium have been reduced more and more along with a remarkable increase in the capacity and recording density of magnetic recording disks. In the magnetic recording medium known at present, it is difficult to attain a super high density recording of 3 Gbit or more per one square inch, and there is a need to further decrease the medium noises. For this purpose, it is important to decrease the crystal grain size of the magnetic layers. However, when the volume of the magnetic particles is reduced extremely by the refinement of the magnetic crystal grains, the effect of the thermal energy becomes remarkable even at normal temperatures, which raises a concern that the recorded magnetization will decay. Actually, it has been reported by Y. Hosoe, et al, that information recorded at a density of 225 kFCI (FCI: Flux Change/Inch) is decayed by as much as 10% or more after 96 hours in a noise-reduced medium (IEEE Trans. Magn. 33, pp, 3028-3030, September 1997).
For making the reduction of the medium noises compatible with an improvement of the heat resistant fluctuation performance, it is effective to decrease the average crystal grain size of the magnetic membrane and, at the same time, suppress the growing of extremely small magnetic particles.
As an example of a magnetic recording medium of this type, it has been proposed, for example, in U.S. Pat. No. 5,693,426, by CMU (Carnegie Mellon University), to produce a magnetic recording medium using an under layer having a B2 (CsCl) structure laminated directly thereon or by way of a Cr underlying film, the magnetic layer thereby making the magnetic crystal grains into a non bi-crystal structure.
FIG. 2 is a view illustrating an epitaxial relationship between an underlayer and a magnetic layer of a magnetic recording medium according to the technique proposed by CMU, which will be explained. FIG. 2 shows a crystal structure for an NiAl underlayer, a Cr underlayer and a Co magnetic layer from below. In FIG. 2, the group on the left illustrates the shape of the crystals in which a meshed plane represents a portion growing in parallel with a substrate, and the group on the right shows a representative size of the meshed plane.
The crystal structure for each of the layers is: B2 for the NiAl underlayer, (b.c.c.) for the Cr underlayer and (h.c.p.) for the Co magnetic layer. When the NiAl underlayer is formed on the substrate while optimizing the deposition condition, crystals grow preferentially such that (211) is in parallel with the substrate. The Cr underlayer formed thereon shows substantial orientation (211) and, further, the magnetic layer shows substantial orientation (10.0).
When atoms are located at lattice points of crystals possessed by each of the layers, when each of the layers has the orientation as described above, a rectangle is formed in a film plane as shown on the left of FIG. 2. As a result, when each of the layers is formed successively on the substrate, a layer structure is obtained in which meshed portions in FIG. 2 are stacked successively. When the sizes of the rectangles are compared, while using the bulk value for the lattice constant of each of the layers, it can be seen that they are substantially of the same size in the [0001] direction of the magnetic layer (direction of c-axis), that is, in the direction of the axis of easy magnetization. On the other hand, when the length of the sizes of the rectangles formed with the respective layers are compared in the direction perpendicular thereto, that is, in the [1-210] direction of the magnetic layer, it can be seen that the sizes are different.
According to the result of an experiment conducted by the present inventors, it has been found that the orientation of the axis of easy magnetization to the in-plane direction can be improved particularly by making the sizes of the respective rectangles formed by the underlayer adjacent to the magnetic layer and the magnetic layer substantially equal to each other. For a medium having the structure proposed by CMU, when the rectangles formed by the Co magnetic layer and the Cr underlayer adjacent to the magnetic layer are compared, the length of the sizes are substantially equal in the [0001] direction of the magnetic layer, but the length for the side of the rectangle formed by the underlayer is excessively small in the [1-210] direction perpendicular thereto. In a case where such a size difference exists, the in-plane orientation of the axis of easy magnetization of the magnetic layer is remarkably deteriorated, resulting in a decrease of the coercivity and an increase in the media noise. Further, for the purpose of increasing the coercivity and the reduction of the media noise, elements such as Pt, Ta, Ti, Nb are added generally to the magnetic layer. Therefore, the unit lattice (lattice constant) of the magnetic layer having the h.c.p. structure, that is, the size of the rectangle formed by the alloy magnetic layer is greater than that of Co, and, in the longitudinal magnetic recording medium having a structure in which a Cr underlayer is formed on the underlayer having the B2 structure of NiAl, etc. proposed by CMU, lattice matching between the Cr underlayer and the Co alloy magnetic layer is further deteriorated, thereby to worsen the in-plane orientation of the axis of easy magnetization.
Since the magnetic recording medium proposed by CMU as described above is a longitudinal recording medium, it is preferred that the axis of easy magnetization of the medium is oriented within a plane for attaining high coercivity and reduced noise. Generally, since the magnetic layer comprises Co as the main ingredient, the crystal structure has a substantially hexagonal closed packed lattice with the direction of the axis of easy magnetization being in the direction of the c axis. Then, in the magnetic recording medium in which a magnetic layer is formed on the B2 (mainly comprising NiAl) underlayer directly or by way of the Cr underlayer proposed by CMU, the axis of easy magnetization of the medium shows an in-plane orientation when the c-axis length of the magnetic layer has a size nearly equal to that of Co. However, in the usual magnetic layer, elements such as Pt, Ta, Ti or Nb are added as described above with an aim of improving the coercivity and reducing the media noise. In this case, the lattice constant of the magnetic layer is made greater compared with that of Co, thereby to bring about a problem in that the matching property with the lattice of the B2 underlayer or the Cr underlayer is deteriorated and the in-plane orientation of the axis of easy magnetization is worsened.
A first object of the present invention is to provide a longitudinal magnetic recording medium of high coercivity, reduced noise and which has an excellent thermal decay resistance, by developing the magnetic recording medium of the structure proposed by CMU and improving the in-plane orientation of the axis of the easy magnetization also for the magnetic layer with the addition of an element such as Pt, Ta, Ti or Nb.
A second object of the present invention is to provide a magnetic storage apparatus having a recording density of 3 Gbit or more per square inch, while fully talking an advantage of the performance of the longitudinal magnetic recording medium.
At first, an explanation will be given of the basic concept of the present invention for solving the problem that the lattice matching between the Cr underlayer and the Co alloy magnetic layer is deteriorated to worsen the in-plane orientation of the axis of easy magnetization in the longitudinal magnetic recording medium of a structure, in which a Cr underlayer is formed on an underlayer having a B2 structure, such as NiAl proposed by CMU.
That is, for solving the foregoing problem, it is important to add an element having an atomic radius larger than that of Cr to the underlayer adjacent with the magnetic layer thereby increasing the length of the side of the rectangle so that it is somewhat larger in the [0001] direction and somewhat smaller in the [1-210] direction of the magnetic layer.
Further, according to the result of the experiment conducted by the present inventors, it has been found that when an underlayer having a lattice constant greater than that of Cr is formed directly on an NiAl underlayer (hereinafter referred to as an NiAl orientation control layer), the lattice matching between the NiAl orientation control layer and the underlayer is deteriorated to worsen the orientation of the underlayer and, simultaneously, make the crystal grain size coarser, resulting in deterioration of the coercivity squareness and an increase in the minimum magnetization reversal volume. Deterioration of the coercive squareness deteriorates the resolution upon high density recording (capability of signal recording), and an increase in the minimum magnetization reversal volume increases the media noise. The Cr underlayer formed on the NiAl orientation control layer put to the (211) orientation tends to show a (211) orientation, for example, by reason of lattice matching or chemical stability. On the other hand, it has been found that since the second underlayer formed on the NiAl orientation control layer oriented in the (211) direction and having a lattice constant greater than that of the first Cr underlayer has a lattice larger than that of the Cr underlayer and contains an element different from Cr, the (110) orientation develops in addition to the (211) orientation for the reason, for example, of lattice matching or chemical stability.
The fundamental structure of the magnetic recording medium according to the present invention has a feature, as shown in FIG. 1, in a dual layer underlayer structure in which a first Cr underlayer is disposed on an NiAl orientation control layer and a second underlayer having a lattice constant greater than that of Cr is disposed thereon. This construction is based on the finding that a high coercivity of the medium can be attained with such a structure without deteriorating the coercivity squareness and with the axis of easy magnetization being oriented in-plane of the magnetic layer.
FIG. 1 is a view showing the structure of a magnetic recording medium based on the basic concept of the present invention relative to the underlayer and the magnetic layer of the magnetic recording medium proposed by CMU, and the figure shows an epitaxial relationship in a case where a first Cr underlayer is disposed on an NiAl orientation control layer and a second underlayer having a lattice constant greater than that of Cr is disposed thereon to provide a dual underlayer structure, which will be explained hereinafter. FIG. 1 shows the crystal structure for an NiAl underlayer, a first Cr underlayer, a second CrTi underlayer and a Co magnetic layer successively from below. The meaning of the meshed portion in FIG. 1 is identical with that shown in FIG. 2.
When the crystallographic orientation of the magnetic recording medium according to the present invention was examined by a xcex8-2xcex8 scan method using an X-ray diffraction device, the B2 orientation control layer did not completely orient in the (211) direction, but contained some (110) component. In this case, the underlayer also contained (211) and some (110) components. From the magnetic layer, intense (10.0) and weak (00.2) and (10.1) were detected. As the feature of the present invention, it is important that (11.0) is not detected as the X-ray diffraction component from the magnetic layer.
A magnetic recording medium having a dual underlayer, in which a second CrMo alloy underlayer is formed on a first Cr underlayer, has been described, for example, as descried in Japanese Published Unexampled Patent Application Hei 7-21543. However, this technique forms the first Cr underlayer directly on the substrate, which is greatly different from the magnetic recording medium according to the present invention, in that an orientation control layer having a B2 structure is not disposed between the substrate and the first underlayer. If the orientation control layer having the B2 structure is not provided, the underlayer orients in the (100) direction and the magnetic layer thereon orients in the (11.0) direction. In this case, while the axis of easy magnetization of the magnetic layer orients in the plane of the layer, it takes a structure in which a plurality of magnetic crystal grains in which the axes of easy magnetization are perpendicular on one of the underlayer crystal grains (bi-crystal structure). When the magnetic layer has such a structure, it is difficult to control the crystal grain size to form crystal grains of extremely small size, which tends to undergo the effect of thermal fluctuation as explained above, and the read output is decreased with lapse of time.
Further, since the Cr segregation effect is small between the grains having the bi-crystal structure, the inter grain action is strengthened and an effective anisotropic energy is decreased to lower the coercivity. Such a phenomenon becomes remarkable particularly in a region of the magnetic layer in which the magnetization is made smaller, and the product of the residual magnetic flux density and the thickness of the magnetic layer is 70 Gxc2x7xcexcm or less to bring about d serious problem. The feature of the present invention for solving such a problem is to epitaxially grow the (10.0) oriented magnetic layer by orienting first and second underlayers in the (211) direction thereby growing one magnetic crystal grain on one underlayer crystal grain, namely, not having a bi-crystal structure.
One of the means adopted for this purpose is the provision of an orientation control layer having a B2 structure between the substrate and the underlayer. For the magnetic recording medium according to the present invention, it has been confirmed by the xcex8-2xcex8 scan method using an X-ray diffraction device that (10.0) of the magnetic layer is detected in a plane parallel with the substrate, but (11.0) is not detected.
As described above, when the orientation control layer having the B2 structure is disposed between the substrate and the underlayer, not only will the preferential orientation face of the underlayer change, but also the fine structure of the magnetic layer is changed, so that it is possible to provide a magnetic recording medium of higher coercivity, lower noise and with an excellent thermal decay resistance compared with the magnetic recording medium having a simple dual underlayer.
The second underlayer in FIG. 1 preferably contains at least one element selected from Cr, Mo and Ti, and has a lattice constant greater than that of Cr and, particularly, preferably comprises a composition of Cr and Ti from 5 at. % to 50 at. % of Ti, Cr and from 5 at. % to 100 at. % of Mo, or Cr, Mo and Ti for increasing the in-plane orientation of the axis of easy magnetization of the magnetic layer. However, it is important that the second underlayer has a crystal structure of b.c.c. The alloy of Cr and Mo used for the second underlayer is in a relation of complete series of solid solution in view of the fact that the phase diagram of bulk metal and the crystal structure of the alloy is always in b.c.c., so that it is easy to handle with and particularly preferred for preparing crystals having an optional size of lattice. Further, in a case of using an alloy of Cr and Ti, since the crystal grains of the underlayer can be made smaller and the crystal grain size of the magnetic layer grown thereon can also be made smaller, it is particularly preferred from the point of view of reducing the noises.
However, since Ti has a crystal structure h.c.p. in the Crxe2x80x94Ti alloy, Ti in the composition of the second underlayer has to be 50 at. % or less based on the entire part. The second underlayer comprising Cr, Mo and Ti succeeds the natures of Crxe2x80x94Mo, Crxe2x80x94Ti in accordance with the concentration of the respective elements. When elements other than Cr, Mo, Ti are used for the second underlayer, it is preferred to use Nb, Ta, Mo (however, the characteristic is somewhat poor compared with Cr, Mo, Ti). Use of other elements than the above is not preferred since the orientation of crystals is distorted, or the crystal grain size is made coarser, resulting in a lowering of the coercivity or an increase in the media noise.
The magnetic layer preferably contains from 15 at. % to 25 at. % of Cr and from 4 at. % to 25 at. % of Pt for increasing the coercivity and reducing the media noise. Further, in a case of adding Ta, Ti, Nb for reducing the noise, it is important to control the concentration for the total of the elements to 8 at. % or less in order to prevent non magnetization of the magnetic layer. In the composition of the magnetic layer, at least Co of 62 at. % or more is necessary. If the Co concentration is 62 at. % or less, the magnetic flux density lowers remarkably to decrease magnetic fluxes which leak from the medium, making it difficult to read out signals with the magnetic head.
When the magnetic layer having a h.c.p. structure is epitaxially grown on an underlayer having a b.c.c. structure, since grains of different kinds of crystal structures are compulsorily subjected to crystal growth, defects are introduced in the initial state of the crystal growth of the magnetic layer or fine magnetic crystal grains are formed. Such defects or fine particles tend to intensely undergo the effect of thermal fluctuation and a decreasing ratio of the read output with elapse of time increases after recording the signals. For minimizing the effect as much as possible, it is preferred to interpose an intermediate layer having a non-magnetic h.c.p. structure between the underlayer and the magnetic layer. The non-magnetic h.c.p. intermediate layer absorbs defects or fine particles formed at the boundary with the b.c.c. underlayer to prevent undesired effects on the magnetic layer. As the material for the non-magnetic h.c.p. intermediate layer, use of a material comprising Co with the addition of at least 25 at. % or more of Cr or a material comprising Co and Ti or Ti as the main ingredient is preferred.
As a result of the consideration described above, the foregoing object of the present invention can be attained by disposing at least an orientation control layer having a B2 structure on a substrate, disposing thereon, a first underlayer comprising Cr and a second underlayer containing at least one element selected from Cr, Nb, Mo, Ta, W and Ti and comprising Cr having a lattice constant greater than that of the first underlayer, and then forming a magnetic layer comprising Co as the main ingredient.
Further, the object of the present invention can be attained by using an alloy comprising, as the main ingredient, at least one member selected from Alxe2x80x94Co, Alxe2x80x94Fe, Alxe2x80x94Ni, Alxe2x80x94Pd, Coxe2x80x94Ga, Coxe2x80x94Fe, Coxe2x80x94Ti. Cixe2x80x94Pd, Cuxe2x80x94Zn, Gaxe2x80x94Ni, Gaxe2x80x94Rh and Ruxe2x80x94Si for the orientation control layer having the B2 structure, in which the crystal grain size can be refined and the in-plane orientation of the axis of easy magnetization of the magnetic layer can be improved.
Further, a magnetic storage apparatus having a recording density of 3 Gbit or more per one square inch can be attained in a magnetic storage apparatus comprising a longitudinal magnetic recording medium according to the present invention, a driving section for driving the longitudinal magnetic medium in a recording direction, a magnetic head comprising a recording section and a read out section, means for relatively moving the magnetic head to the longitudinal magnetic recording medium and a recording/reading signal processing means for applying waveform processing to input signals and output signals relative to the magnetic head, by constituting the read out section of the magnetic head with a magnetoresistive head.